Semiconductor device, method for manufacturing semiconductor device, inverter circuit, driving device, vehicle, and elevator

ABSTRACT

A semiconductor device according to an embodiment includes a SiC layer having a first and a second plane; a first electrode having a first region in the SiC layer, the inclination angle of a side surface of the first region being 60 to 85 degrees; a second electrode; a first gate electrode; a second gate electrode facing the first gate electrode; first and second gate insulating layers; a first region of a first conductivity type in the SiC layer; a second region of a second conductivity type between the first region and the first gate insulating layer; a third region of the second conductivity type between the first region and the second gate insulating layer; a sixth region of the second conductivity type between the first region and the first region; and a seventh region of the second conductivity type between the first region and the sixth region.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2016-144515, filed on Jul. 22, 2016, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor device,a method for manufacturing a semiconductor device, an inverter circuit,a driving device, a vehicle, and an elevator.

BACKGROUND

Silicon carbide (SiC) is expected to be used as a material for anext-general semiconductor device. SiC has better physical propertiesthan silicon (Si). For example, SiC has a bandgap that is three timeswider than that of Si, a breakdown field strength that is about tentimes higher than that of Si, and a thermal conductivity that is aboutthree times higher than that of Si. These physical properties are usedto achieve a semiconductor device which has low loss and can operate ata high temperature.

However, for example, when silicon carbide is used to form a metalinsulator semiconductor (MIS) structure, there is a concern that thebreakdown voltage of a gate insulating layer will become lower than thebreakdown voltage of a semiconductor, since the breakdown voltage ofsilicon carbide is high. In particular, when the MIS structure is formedin a trench in order to increase an integrity of a device, the breakdownvoltage of the gate insulating layer is reduced due to the concentrationof the electric field at the bottom of the trench.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating asemiconductor device according to a first embodiment.

FIG. 2 is a cross-sectional view schematically illustrating thesemiconductor device that is being manufactured in a semiconductordevice manufacturing method according to the first embodiment.

FIG. 3 is a cross-sectional view schematically illustrating thesemiconductor device that is being manufactured in the semiconductordevice manufacturing method according to the first embodiment.

FIG. 4 is a cross-sectional view schematically illustrating thesemiconductor device that is being manufactured in the semiconductordevice manufacturing method according to the first embodiment.

FIG. 5 is a cross-sectional view schematically illustrating thesemiconductor device that is being manufactured in the semiconductordevice manufacturing method according to the first embodiment.

FIG. 6 is a cross-sectional view schematically illustrating thesemiconductor device that is being manufactured in the semiconductordevice manufacturing method according to the first embodiment.

FIG. 7 is a cross-sectional view schematically illustrating asemiconductor device according to a comparative example.

FIG. 8 is a diagram illustrating the function and effect of thesemiconductor device according to the first embodiment.

FIG. 9 is a cross-sectional view schematically illustrating thesemiconductor device which is being manufactured in a semiconductordevice manufacturing method according to the comparative example.

FIG. 10 is a cross-sectional view schematically illustrating thesemiconductor device which is being manufactured in the semiconductordevice manufacturing method according to the comparative example.

FIG. 11 is a cross-sectional view schematically illustrating thesemiconductor device which is being manufactured in the semiconductordevice manufacturing method according to the comparative example.

FIG. 12 is a cross-sectional view schematically illustrating thesemiconductor device which is being manufactured in the semiconductordevice manufacturing method according to the comparative example.

FIG. 13 is a cross-sectional view schematically illustrating thesemiconductor device which is being manufactured in the semiconductordevice manufacturing method according to the comparative example.

FIG. 14 is a diagram illustrating the function and effect of thesemiconductor device according to the first embodiment.

FIG. 15 is a diagram illustrating the function and effect of thesemiconductor device according to the first embodiment.

FIG. 16 is a cross-sectional view schematically illustrating amodification example of the semiconductor device according to the firstembodiment.

FIG. 17 is a cross-sectional view schematically illustrating asemiconductor device according to a second embodiment.

FIG. 18 is a cross-sectional view schematically illustrating asemiconductor device according to a third embodiment.

FIG. 19 is a diagram schematically illustrating a driving deviceaccording to a fourth embodiment.

FIG. 20 is a diagram schematically illustrating a vehicle according to afifth embodiment.

FIG. 21 is a diagram schematically illustrating a vehicle according to asixth embodiment.

FIG. 22 is a diagram schematically illustrating an elevator according toa seventh embodiment.

DETAILED DESCRIPTION

A semiconductor device according to an embodiment includes a siliconcarbide layer having a first plane and a second plane; a first electrodehaving a first region provided in the silicon carbide layer, a width ofa second-plane-side end portion of the first region being less than awidth of a first-plane-side end portion of the first region, a firstinclination angle of a side surface of the first region with respect toa plane parallel to the first plane being equal to or greater than 60degrees and equal to or less than 85 degrees; a second electrode facingthe first electrode, the silicon carbide layer being interposed betweenthe second electrode and the first electrode; a first gate electrode; asecond gate electrode facing the first gate electrode, the first regionbeing interposed between the second gate electrode and the first gateelectrode; a first gate insulating layer which is provided between thefirst region and the first gate electrode; a second gate insulatinglayer which is provided between the first region and the second gateelectrode; a first silicon carbide region of a first conductivity typewhich is provided in the silicon carbide layer; a second silicon carbideregion of a second conductivity type which is provided between the firstsilicon carbide region and the first plane and between the first regionand the first gate insulating layer; a third silicon carbide region ofthe second conductivity type which is provided between the first siliconcarbide region and the first plane and between the first region and thesecond gate insulating layer; a fourth silicon carbide region of thefirst conductivity type which is provided between the second siliconcarbide region and the first plane; a fifth silicon carbide region ofthe first conductivity type which is provided between the third siliconcarbide region and the first plane and faces the fourth silicon carbideregion, with the first region interposed therebetween; a sixth siliconcarbide region of the second conductivity type which is provided betweenthe second-plane-side end portion of the first region and the firstsilicon carbide region and between the side surface of the first regionand the first silicon carbide region and has a highersecond-conductivity-type impurity concentration than the second siliconcarbide region and the third silicon carbide region; and a seventhsilicon carbide region of the second conductivity type which is providedbetween the first silicon carbide region and the sixth silicon carbideregion and has a lower second-conductivity-type impurity concentrationthan the sixth silicon carbide region, a distance between the seventhsilicon carbide region and the second plane being less than a distancebetween the second plane and the second silicon carbide region and adistance between the second plane and the third silicon carbide region.

Hereinafter, embodiments of the invention will be described withreference to the drawings. In the following description, the same orsimilar members are denoted by the same reference numerals and thedescription thereof will not be repeated.

In the following description, n⁺, n, n⁻, p⁺, p, and p⁻ indicate therelative levels of impurity concentration in each conductivity type.That is, n⁺ indicates an n-type impurity concentration which is higherthan that of n and n⁻ indicates an n-type impurity concentration whichis lower than that of n. In addition, p⁺ indicates a p-type impurityconcentration which is higher than that of p and p⁻ indicates a p-typeimpurity concentration which is lower than that of p. In some cases, ann⁺ type and an n⁻ type are simply referred to as an n type and a p⁺ typeand a p⁻ type are simply referred to as a p type.

First Embodiment

A semiconductor device according to this embodiment includes a siliconcarbide layer having a first plane and a second plane; a first electrodehaving a first region provided in the silicon carbide layer, a width ofa second-plane-side end portion of the first region being less than awidth of a first-plane-side end portion of the first region, a firstinclination angle of a side surface of the first region with respect toa plane parallel to the first plane being equal to or greater than 60degrees and equal to or less than 85 degrees; a second electrode whichfaces the first electrode, with the silicon carbide layer interposedtherebetween; a first gate electrode; a second gate electrode whichfaces the first gate electrode, with the first region interposedtherebetween; a first gate insulating layer which is provided betweenthe first region and the first gate electrode; a second gate insulatinglayer which is provided between the first region and the second gateelectrode; a first silicon carbide region of a first conductivity typewhich is provided in the silicon carbide layer; a second silicon carbideregion of a second conductivity type which is provided between the firstsilicon carbide region and the first plane and between the first regionand the first gate insulating layer; a third silicon carbide region ofthe second conductivity type which is provided between the first siliconcarbide region and the first plane and between the first region and thesecond gate insulating layer; a fourth silicon carbide region of thefirst conductivity type which is provided between the second siliconcarbide region and the first plane; a fifth silicon carbide region ofthe first conductivity type which is provided between the third siliconcarbide region and the first plane and faces the fourth silicon carbideregion, with the first region interposed therebetween; a sixth siliconcarbide region of the second conductivity type which is provided betweenthe second-plane-side end portion of the first region and the firstsilicon carbide region and between the side surface of the first regionand the first silicon carbide region and has a highersecond-conductivity-type impurity concentration than the second siliconcarbide region and the third silicon carbide region; and a seventhsilicon carbide region of the second conductivity type which is providedbetween the first silicon carbide region and the sixth silicon carbideregion and has a lower second-conductivity-type impurity concentrationthan the sixth silicon carbide region, a distance between the seventhsilicon carbide region and the second plane being less than a distancebetween the second plane and the second silicon carbide region and adistance between the second plane and the third silicon carbide region.

FIG. 1 is a cross-sectional view schematically illustrating thesemiconductor device according to this embodiment. A metal oxidesemiconductor field effect transistor (MOSFET) 100 is, for example, adouble implantation MOSFET (DIMOSFET) in which a well region and asource region are formed by ion implantation. In addition, the MOSFET100 is a trench gate MOSFET in which a gate insulating layer and a gateelectrode are provided in a trench. Furthermore, the MOSFET 100 is adouble trench MOSFET in which a portion of a source electrode isprovided in a trench.

FIG. 1 is a cross-sectional view illustrating a portion of the MOSFET100. The MOSFET 100 has a structure in which a pattern illustrated inFIG. 1 is repeatedly arranged.

Hereinafter, an example in which a first conductivity type is an n typeand a second conductivity type is a p type will be described. The MOSFET100 is an n-type MOSFET having electrons as carriers.

The MOSFET 100 includes a silicon carbide layer (SiC layer) 10, a sourceelectrode 12, a drain electrode 14, a first gate insulating layer 16 a,a second gate insulating layer 16 b, a first gate electrode 18 a, asecond gate electrode 18 b, an interlayer insulating film 20, a firstgate trench 50 a, a second gate trench 50 b, and a contact trench 52.The SiC layer 10 includes a SiC substrate 22, a drift region (firstsilicon carbide region) 24, a first p well region (second siliconcarbide region) 26 a, a second p well region (third silicon carbideregion) 26 b, a first source region (fourth silicon carbide region) 28a, a second source region (fifth silicon carbide region) 28 b, a contactregion (sixth silicon carbide region) 32, and an electric field reducingregion (seventh silicon carbide region) 34.

The SiC layer 10 is, for example, a 4H-SiC single-crystal layer.

SiC can have a plurality of crystal forms. Examples of SiC include4H-SiC which is a hexagonal crystal system, 6H-SiC which is a hexagonalcrystal system, and 3C-SiC which is a cubic crystal system. Thearrangement of atoms in SiC can be observed by, for example, atransmission electron microscope (TEM) to identify the crystal form ofSiC. In addition, the arrangement of atoms in SiC can be observed by,for example, X-ray diffraction (XRD) to identify the crystal form ofSiC.

The SiC layer 10 has a first plane and a second plane. In FIG. 1, thefirst plane is an upper plane and the second plane is a lower plane.Hereinafter, the first plane is referred to as a front surface and thesecond plane is referred to as a rear surface. The SiC layer 10 isinterposed between the source electrode 12 and the drain electrode 14.

An example in which the first plane is inclined at an angle that isequal to or greater than 0 degrees and equal to or less than 8 degreeswith respect to a (0001) face and the second plane is inclined at anangle that is equal to or greater than 0 degrees and equal to or lessthan 8 degrees with respect to a (000-1) face will be described. The(0001) face is referred to as a silicon face. The (000-1) face isreferred to as a carbon face.

The source electrode 12 is provided on the front surface of the SiClayer 10. The source electrode 12 includes a trench source region (firstregion) 12 a that is buried in the contact trench 52. The trench sourceregion 12 a is provided so as to come into contact with the side andbottom of the contact trench 52. The trench source region 12 a isprovided in the SiC layer 10.

In other words, a recess is provided in the front surface of the SiClayer 10. The source electrode 12 has a protrusion. A side surface ofthe protrusion of the source electrode 12 comes into contact with theinner side of the recess.

The width (“W1” in FIG. 1) of a second-plane-side end portion of thetrench source region 12 a is less than the width (“W2” in FIG. 1) of afirst-plane-side end portion of the trench source region 12 a. A firstinclination angle (“θ1” in FIG. 1) of the side surface of the trenchsource region 12 a with respect to a plane parallel to the first planeis equal to or greater than 60 degrees and equal to or less than 85degrees. When the first inclination angle of the side surface of thetrench source region 12 a is not constant, for example, a firstinclination angle at the depth of an intermediate position between thefirst plane and the second-plane-side end portion of the trench sourceregion 12 a is used as a representative value of the first inclinationangle of the side surface of the trench source region 12 a.

The width (“W1” in FIG. 1) of the second-plane-side end portion of thetrench source region 12 a is, for example, equal to or greater than 0.4μm and equal to or less than 1.2 μm. The width (“W2” in FIG. 1) of thefirst-plane-side end portion of the trench source region 12 a is, forexample, equal to or greater than 0.6 μm and equal to or less than 1.4μm. The depth of the trench source region 12 a is, for example, equal toor greater than 0.4 μm and equal to or less than 1.2 μm.

In the specification, the “depth” means the distance from the frontsurface of the SiC layer 10.

The distance (“S” in FIG. 1) between the trench source region 12 a andthe first gate insulating layer 16 a is, for example, equal to orgreater than 0.1 μm and equal to or less than 0.8 μm. In other words,the distance (“S” in FIG. 1) between the first gate insulating layer 16a and a first contact point between a side surface of the trench sourceregion 12 a which is close to the first gate insulating layer 16 a and afirst plane P1 is, for example, equal to or greater than 0.1 μm andequal to or less than 0.8 μm.

The distance between the trench source region 12 a and the second gateinsulating layer 16 b is, for example, equal to or greater than 0.1 μmand equal to or less than 0.8 μm. In other words, the distance betweenthe second gate insulating layer 16 b and a second contact point betweena side surface of the trench source region 12 a which is close to thesecond gate insulating layer 16 b and the first plane P1 is, forexample, equal to or greater than 0.1 μm and equal to or less than 0.8μm.

The source electrode 12 is electrically connected to the first sourceregion 28 a, the second source region 28 b, and the contact region 32.The source electrode 12 comes into contact with the first source region28 a, the second source region 28 b, and the contact region 32. Thesource electrode 12 has a function of applying potential to the firstsource region 28 a, the second source region 28 b, and the contactregion 32.

The source electrode 12 is made of metal. The metal forming the sourceelectrode 12 has, for example, a stacked structure of titanium (Ti) andaluminum (Al). The metal forming the source electrode 12 may react withthe SiC layer 10 to form metal silicide or metal carbide.

The drain electrode 14 is provided on the rear surface of the SiC layer10. The drain electrode 14 is electrically connected to the SiCsubstrate 22. The drain electrode 14 is stacked on the rear surface sideof the SiC layer 10.

The drain electrode 14 is made of metal. The metal forming the drainelectrode 14 is, for example, nickel silicide.

The SiC substrate 22 is made of n⁺ SiC. The SiC substrate 22 includes,for example, nitrogen (N) as n-type impurities. The n-type impurityconcentration of the SiC substrate 22 is, for example, equal to orgreater than 1×10¹⁸ cm⁻³ and equal to or less than 1×10²¹ cm⁻³.

The n-type impurity concentration of the SiC substrate 22 in the secondplane is preferably equal to or greater than 1×10¹⁹ cm⁻³ and morepreferably equal to or greater than 1×10²⁰ cm⁻³ in order to reduce thecontact resistance between the drain electrode 14 and the SiC substrate22.

The drift region 24 is provided on the SiC substrate 22. The driftregion 24 is, for example, an n⁻ SiC region that is formed on the SiCsubstrate 22 by epitaxial growth. The thickness of the drift region 24is, for example, equal to or greater than 5 μm and equal to or less than150 μm.

The drift region 24 includes, for example, nitrogen (N) as n-typeimpurities. The n-type impurity concentration of the drift region 24 islower than the n-type impurity concentration of the SiC substrate 22.The n-type impurity concentration of the drift region 24 is, forexample, equal to or greater than 1×10¹⁴ cm⁻³ and equal to or less than5×10¹⁷ cm⁻³.

The first p well region 26 a and the second p well region 26 b areprovided between the drift region 24 and the first plane. The first pwell region 26 a and the second p well region 26 b are made of p-typeSiC.

The first p well region 26 a is provided between the first source region28 a and the drift region 24. The second p well region 26 b is providedbetween the second source region 28 b and the drift region 24.

The first p well region 26 a is provided between the trench sourceregion 12 a and the first gate insulating layer 16 a. The second p wellregion 26 b is provided between the trench source region 12 a and thesecond gate insulating layer 16 b.

The first p well region 26 a and the second p well region 26 b functionas the channel regions of the MOSFET 100.

The first p well region 26 a and the second p well region 26 b are madeof p-type SiC. The first p well region 26 a and the second p well region26 b include, for example, aluminum (Al) as p-type impurities. Thep-type impurity concentration of the first p well region 26 a and thesecond p well region 26 b is, for example, equal to or greater than5×10⁵ cm⁻³ and equal to or less than 1×10¹⁸ cm⁻³.

The distance between the top and the bottom of the first p well region26 a is, for example, equal to or greater than 0.2 μm and equal to orless than 0.6 μm. The distance between the top and the bottom of thesecond p well region 26 b is, for example, equal to or greater than 0.2μm and equal to or less than 0.6 μm.

The first source region 28 a is provided between the first p well region26 a and the first plane. The second source region 28 b is providedbetween the second p well region 26 b and the first plane of the SiClayer 10. The trench source region 12 a is interposed between the firstsource region 28 a and the second source region 28 b.

The first source region 28 a and the second source region 28 b are madeof n⁺ SiC. The first source region 28 a and the second source region 28b include, for example, phosphorus (P) as n-type impurities.

The n-type impurity concentration of the first source region 28 a andthe second source region 28 b is higher than the n-type impurityconcentration of the drift region 24. The n-type impurity concentrationof the first source region 28 a and the second source region 28 b is,for example, equal to or greater than 1×10¹⁹ cm⁻³ and equal to or lessthan 1×10²¹ cm⁻³.

It is preferable that the n-type impurity concentration of the firstsource region 28 a and the second source region 28 b in the first planebe equal to or greater than 1×10²⁰ cm⁻³ in order to reduce the contactresistance between the source electrode 12 and the first and secondsource regions 28 a and 28 b.

The depth of the first source region 28 a and the second source region28 b is less than the depth of the bottom of the first p well region 26a and the second p well region 26 b and is, for example, equal to orgreater than 0.05 μm and equal to or less than 0.5 μm.

The first gate trench 50 a and the second gate trench 50 b are providedin the SiC layer 10 so as to extend from the first plane to the secondplane of the SiC layer 10. The depth of the first gate trench 50 a andthe second gate trench 50 b is greater than the depth of the first pwell region 26 a and the second p well region 26 b. The depth of thefirst gate trench 50 a and the second gate trench 50 b is, for example,equal to or greater than 0.4 μm and equal to or less than 1.0 μm.

The first gate insulating layer 16 a is provided in the first gatetrench 50 a. The first gate electrode 18 a is provided on the first gateinsulating layer 16 a in the first gate trench 50 a.

The first gate insulating layer 16 a is provided between the trenchsource region 12 a and the first gate electrode 18 a.

The second gate insulating layer 16 b is provided in the second gatetrench 50 b. The second gate electrode 18 b is provided on the secondgate insulating layer 16 b in the second gate trench 50 b.

The second gate insulating layer 16 b is provided between the trenchsource region 12 a and the second gate electrode 18 b.

Each of the first gate insulating layer 16 a and the second gateinsulating layer 16 b is, for example, a silicon oxide film. Thethickness of the first gate insulating layer 16 a and the second gateinsulating layer 16 b is, for example, equal to or greater than 40 nmand equal to or less than 60 nm.

The trench source region 12 a is interposed between the first gateelectrode 18 a and the second gate electrode 18 b.

The first gate electrode 18 a and the second gate electrode 18 b aremade of, for example, polysilicon including n-type impurities or p-typeimpurities.

The contact region 32 is provided so as to come into contact with theside and bottom of the contact trench 52. The contact region 32 comesinto contact with the side surface of the trench source region 12 a andthe second-plane-side end portion of the trench source region 12 a. Thetop of the contact region 32 comes into contact with, for example, thefirst source region 28 a and the second source region 28 b.

The contact region 32 is made of p⁺ SiC. The contact region 32 includes,for example, aluminum (Al) as p-type impurities.

The p-type impurity concentration of the contact region 32 is higherthan the p-type impurity concentration of the first p well region 26 aand the second p well region 26 b. The p-type impurity concentration ofthe contact region 32 is, for example, equal to or greater than 1×10¹⁹cm⁻³ and equal to or less than 1×10 cm⁻³.

The electric field reducing region 34 is provided around the contacttrench 52. The electric field reducing region 34 is provided between thedrift region 24 and the contact region 32.

The second inclination angle (“θ2” in FIG. 1) of the boundary betweenthe electric field reducing region 34 and the drift region 24 withrespect to the plane parallel to the first plane is, for example, equalto or greater than 60 degrees and equal to or less than 85 degrees. Inaddition, when the second inclination angle of the boundary between theelectric field reducing region 34 and the drift region 24 is notconstant, for example, a second inclination angle at the depth of thesame position as the second-plane-side end portion of the trench sourceregion 12 a is used as a representative value of the second inclinationangle of the side surface of the trench source region 12 a.

The distance (“d1” in FIG. 1) between the second plane and the electricfield reducing region 34 is less than the distance (“d2” in FIG. 1)between the second plane and the first p well region 26 a and thedistance between the second plane and the second p well region 26 b. Inother words, the depth of the electric field reducing region 34 isgreater than the depth of the first p well region 26 a and the second pwell region 26 b.

The distance (“d1” in FIG. 1) between the second plane and the electricfield reducing region 34 is less than the distance (“d3” in FIG. 1)between the second plane and the first gate insulating layer 16 a andthe distance between the second plane and the second gate insulatinglayer 16 b. In other words, the depth of the electric field reducingregion 34 is greater than the depth of the first gate trench 50 a andthe second gate trench 50 b.

The electric field reducing region 34 comes into contact with the firstp well region 26 a and the second p well region 26 b.

The electric field reducing region 34 is made of p-type SiC. Theelectric field reducing region 34 includes, for example, aluminum (Al)as p-type impurities.

The p-type impurity concentration of the electric field reducing region34 is lower than the p-type impurity concentration of the contact region32. The p-type impurity concentration of the electric field reducingregion 34 is, for example, equal to or greater than 2×10¹⁷ cm⁻³ andequal to or less than 2×10¹⁸ cm⁻³.

The p-type impurity concentration of the electric field reducing region34 is higher than, for example, the p-type impurity concentration of thefirst p well region 26 a and the second p well region 26 b. For example,the p-type impurity concentration of the electric field reducing region34 is equal to or more than two times the p-type impurity concentrationof the first p well region 26 a and the second p well region 26 b.

The difference between the distance (“d4” in FIG. 1) between the secondplane and the trench source region 12 a and the distance (“d1” inFIG. 1) between the second plane and the electric field reducing region34 is, for example, equal to or less than 1 μm.

The interlayer insulating film 20 is provided on the gate electrode 18.The interlayer insulating film 20 is, for example, a silicon oxide film.

The concentration and distribution of impurities included in the SiClayer 10 can be measured by, for example, a secondary ion massspectroscopy (SIMS). In addition, the relative level of impurityconcentration can be determined from the level of carrier concentrationwhich is calculated by, for example, scanning capacitance microscopy(SCM). For example, the depth of regions including impurities and thedistance between the regions can be calculated by, for example, SIMS.For example, the distance between the region including impurities andthe gate insulating layer can be calculated from a composite image of anSCM image and an atomic force microscope (AFM) image.

A method for manufacturing a semiconductor device according to thisembodiment includes: forming a second region of a second conductivitytype in a silicon carbide layer including a first region of a firstconductivity type and having a first plane and a second plane; formingtwo first trenches in the first plane of the silicon carbide layer so asto be deeper than the second region; forming a second trench in thefirst plane of the silicon carbide layer between the two first trenches,using a mask member covering the two first trenches as a mask, such thatthe second trench is deeper than the second region and an inclinationangle of a side surface thereof with respect to the first plane is equalto or greater than 60 degrees and equal to or less than 85 degrees;implanting ions from the side and bottom of the second trench at anangle of 1 degree or less with respect to a line normal to the firstplane to form a third region of the second conductivity type; implantingions from the side and bottom of the second trench at an angle of 1degree or less with respect to the line normal to the first plane toform a fourth region of the second conductivity type lower than thethird region and having a higher second-conductivity-type impurityconcentration than the third region; forming a gate insulating layer inthe first trench; forming a gate electrode on the gate insulating layerin the first trench; forming a first electrode filling the secondtrench; and forming a second electrode on the second plane.

Hereinafter, an example of the semiconductor device manufacturing methodaccording to this embodiment will be described. FIGS. 2 to 6 arecross-sectional views schematically illustrating the semiconductordevice which is being manufactured in the semiconductor devicemanufacturing method according to this embodiment.

First, an n-type SiC substrate having a first plane which is a siliconface and a second plane which is a carbon face is prepared. The SiCsubstrate becomes the SiC substrate 22. The n-type SiC substrate is a4H-SiC substrate.

Then, the n⁻ drift region (first region) 24 is formed on the first planeof the n-type SiC substrate by an epitaxial growth method. The SiCsubstrate and the n⁻ drift region 24 form the SiC layer 10.

Then, aluminum (Al) ions which are p-type impurity ions are selectivelyimplanted into the drift region 24 by photolithography and an ionimplantation method. The p well region (second region) 26 is formed bythe ion implantation.

Then, phosphorus (P) ions which are n-type impurity ions are selectivelyimplanted into the p well region 26 by photolithography and an ionimplantation method. An n⁺ source region 28 is formed by the ionimplantation (FIG. 2).

Then, two trenches, that is, the first gate trench (first trench) 50 aand the second gate trench (second trench) 50 b are formed in the firstplane of the SiC layer 10. The first gate trench 50 a and the secondgate trench 50 b are formed by, for example, anisotropic dry etchingusing a patterned mask member as a mask. The depth of the first gatetrench 50 a and the second gate trench 50 b is greater than the depth ofthe p well region 26.

Then, a mask member 54 that covers at least the first gate trench 50 aand the second gate trench 50 b is formed. The mask member 54 is formedby, for example, the deposition of a film by a vapor deposition method,lithography, and dry etching. The mask member 54 is, for example, asilicon oxide film.

Then, the contact trench (second trench) 52 is formed in the firstplane, using the mask member 54 as a mask (FIG. 3). The contact trench52 is formed between the first gate trench 50 a and the second gatetrench 50 b. The depth of the contact trench 52 is greater than thedepth of the p well region 26. The contact trench 52 is formed such thatthe first inclination angle (“θ1” in FIG. 3) of the side surface of thecontact trench 52 with respect to the first plane is equal to or greaterthan 60 degrees and equal to or less than 85 degrees.

The contact trench 52 is formed by, for example, anisotropic dryetching. The etching conditions of the anisotropic dry etching can becontrolled to adjust the inclination angle of the side surface of thecontact trench 52 to a desired angle.

The depth of the contact trench 52 is greater than, for example, thedepth of the first gate trench 50 a and the second gate trench 50 b.

Then, aluminum (Al) ions which are p-type impurity ions are implantedfrom the side and bottom of the contact trench 52 into the SiC layer 10,using the mask member 54 as a mask (FIG. 4). The p-type electric fieldreducing region (third region) 34 is formed by the ion implantation.

The ion implantation is performed under the condition that aninclination angle with respect to the line normal to the first plane isequal to or less than 1 degree. Hereinafter, the ion implantationperformed under the condition that the inclination angle with respect tothe line normal to the first plane is equal to or less than 1 degree isreferred to as vertical ion implantation. For example, the ionimplantation is performed under the condition that the p-type impurityconcentration of the electric field reducing region 34 is higher thanthat of the p well region 26.

The boundary between the electric field reducing region 34 and the driftregion 24 is substantially parallel to the side surface of the contacttrench 52. The second inclination angle (“θ2” in FIG. 4) of the boundarybetween the electric field reducing region 34 and the drift region 24with respect to the plane parallel to the first plane is substantiallyequal to the first inclination angle (“θ1” in FIG. 4) of the sidesurface of the contact trench 52 with respect to the first plane and is,for example, equal to or greater than 60 degrees and equal to or lessthan 85 degrees.

Then, aluminum (Al) ions which are p-type impurity ions are implantedfrom the side and bottom of the contact trench 52 into the SiC layer 10,using the mask member 54 as a mask (FIG. 5). The p-type contact region(fourth region) 32 is formed by the ion implantation.

The ion implantation is performed under the condition that aninclination angle with respect to the line normal to the first plane isequal to or less than 1 degree. For example, the ion implantation isperformed under the condition that the p-type impurity concentration ofthe p-type contact region 32 is higher than that of the electric fieldreducing region 34.

Then, a heat treatment for activating the p-type impurities and then-type impurities introduced into the SiC layer 10 by the ionimplantation is performed. The heat treatment is performed in, forexample, a non-oxidizing atmosphere.

The p well region 26 becomes the first p well region 26 a and the secondp well region 26 b. The source region 28 becomes the first source region28 a and the second source region 28 b.

Then, the mask member 54 is removed. Then, the gate insulating layer 16,the gate electrode 18, and the interlayer insulating film 20 are formedby a known process technique (FIG. 6).

Then, the source electrode 12 is formed on the front surface of the SiClayer 10 by a known process technique. The source electrode 12 is formedso as to fill the contact trench 52. In addition, the drain electrode 14is formed on the rear surface of the SiC layer 10.

The MOSFET 100 illustrated in FIG. 1 is formed by the above-mentionedmanufacturing method.

Next, the function and effect of the semiconductor device according tothis embodiment will be described.

In the MOSFET 100 according to this embodiment, the electric fieldreducing region 34 is provided around the contact trench 52 with aninclined side surface such that a pn junction is inclined. According tothis structure, it is possible to improve the breakdown voltage of thegate insulating layer and to reduce on-resistance.

The contact region 32 with a high p-type impurity concentration isprovided around the contact trench 52. According to this structure, itis possible to improve the secondary breakdown resistance of the MOSFET100.

The trench gate MOSFET has the problem that, when the MOSFET is turnedoff, the electric field is concentrated on the bottom of the trench andthe breakdown voltage of the gate insulating layer is reduced. Inparticular, the electric field is concentrated on the corners of thetrench and the breakdown voltage of the gate insulating layer isreduced, which results in a reduction in the breakdown voltage of theMOSFET.

FIG. 7 is a cross-sectional view schematically illustrating asemiconductor device according to a comparative example. A MOSFET 900according to the comparative example differs from the MOSFET 100according to this embodiment in that the side surface of a contacttrench 52 is perpendicular to the first plane. In addition, the MOSFET900 according to the comparative example differs from the MOSFET 100according to this embodiment in that the side surface of the contacttrench 52 does not have the contact region 32.

The MOSFET 900 according to the comparative example includes an electricfield reducing region 34 that is deeper than the first p well region 26a and the second p well region 26 b, similarly to the MOSFET 100according to this embodiment.

When a reverse bias is applied, the electric field is also concentratedon the electric field reducing region 34 and the concentration of theelectric field on the corners of the trench is reduced. Therefore, themaximum electric field strength of the first gate insulating layer 16 aand the second gate insulating layer 16 b is reduced. As a result, thebreakdown voltage of the first gate insulating layer 16 a and the secondgate insulating layer 16 b is improved.

However, in the MOSFET 900, the side surface of the contact trench 52 isperpendicular to the first plane.

Therefore, the width of a portion of the drift region 24 which isinterposed between the first gate insulating layer 16 a and the electricfield reducing region 34 is reduced. A portion of the drift region 24which is interposed between the first gate insulating layer 16 a and theelectric field reducing region 34 is a region surrounded by a dashedline in FIG. 7. Therefore, a current path from a channel region to thedrain electrode 14 is narrowed, which results in an increase in theon-resistance of the MOSFET 900.

FIG. 8 is a diagram illustrating the function and effect of thisembodiment. FIG. 8 illustrates the relationship along the firstinclination angle (“θ1” in FIG. 1) of the side surface of the contacttrench 52, the on-resistance (a black circle in FIG. 8) of the MOSFET100, and the maximum electric field strength applied to the gateinsulating layer (a white circle in FIG. 8).

FIG. 8 illustrates the simulation results. In the simulation, the firstinclination angle (“θ1” in FIG. 1) of the side surface of the contacttrench 52 is a variable and the width (“W2” in FIG. 1) of the contacttrench 52 changes following a change in the first inclination angle.

The second inclination angle (“θ2” in FIG. 1) of the boundary betweenthe electric field reducing region 34 and the drift region 24 withrespect to the plane parallel to the first plane is the same as thefirst inclination angle (“θ1” in FIG. 1) of the side surface of thecontact trench 52 with respect to the first plane.

As can be seen from FIG. 8, as the inclination angle increases, themaximum electric field strength decreases. The reason is that, since theelectric field reducing region 34 is close to the first gate trench 50 aand the second gate trench 50 b, the effect of reducing the electricfield of the gate insulating layer is large.

In contrast, for on-resistance, in an inclination angle range of greaterthan 70 degrees, the on-resistance increases as the inclination angleincreases. The reason is that, since the electric field reducing region34 is close to the first gate trench 50 a and the second gate trench 50b, a current path is narrowed.

Furthermore, in an inclination angle range of less than 70 degrees, theon-resistance increases as the inclination angle decreases. The reasonis that, as the width of the contact trench 52 increases, a cell pitchincreases.

It is preferable that the first inclination angle of the side surface ofthe contact trench 52 be equal to or greater than 60 degrees and equalto or less than 85 degrees, in order to improve the breakdown voltage ofthe gate insulating layer and to reduce on-resistance. In addition, thefirst inclination angle is more preferably equal to or greater than 65degrees and equal to or less than 80 and most preferably equal to orgreater than 70 degrees and equal to or less than 75 degrees.

From the same point of view as described above, the second inclinationangle of the boundary between the electric field reducing region 34 andthe drift region 24 is preferably equal to or greater than 60 degreesand equal to or less than 85 degrees. The second inclination angle ismore preferably equal to or greater than 65 degrees and equal to or lessthan 80 degrees and most preferably equal to or greater than 70 degreesand equal to or less than 75 degrees.

It is preferable that the depth of the electric field reducing region 34be greater than the depth of the first gate trench 50 a and the secondgate trench 50 b in order to increase the effect of reducing theelectric field of the gate insulating layer.

In the MOSFET 100 according to this embodiment, the distance (“S” inFIG. 1) between the trench source region 12 a and the first gateinsulating layer 16 a is preferably equal to or greater than 0.1 μm andequal to or less than 0.8 μm and more preferably equal to or greaterthan 0.3 μm and equal to or less than 0.6 μm. Similarly, the distancebetween the trench source region 12 a and the second gate insulatinglayer 16 b is preferably equal to or greater than 0.1 μm and equal to orless than 0.8 μm and more preferably equal to or greater than 0.3 μm andequal to or less than 0.6 μm. When the distance is less than theabove-mentioned range, there is a concern that a current path will benarrowed and the on-resistance of the MOSFET 100 will increase. Inaddition, when the distance is greater than the range, there is aconcern that a cell pitch will increase and the on-resistance of theMOSFET 100 will increase.

In the MOSFET 100 according to this embodiment, the contact region 32with a high p-type impurity concentration is provided around the contacttrench 52. The contact area of the contact region 32 with the sourceelectrode 12 is greater than that in the MOSFET 900 according to thecomparative example. Therefore, the electric resistance between thesource electrode 12 and the electric field reducing region 34 is lowerthan that in the MOSFET 900.

In a case in which avalanche breakdown occurs when a reverse bias isapplied to the MOSFET 100, holes are transiently accumulated in theelectric field reducing region 34. The avalanche breakdown is alsoreferred to as primary breakdown.

When the electric resistance between the source electrode 12 and theelectric field reducing region 34 is high, the potential of the electricfield reducing region 34 is reduced by the accumulated holes. Then, aparasitic bipolar transistor formed by the source region, the electricfield reducing region, the drift region is turned on and there is aconcern that secondary breakdown will occur. When the secondarybreakdown is large, a large amount of current flows. As a result, thereis a concern that the MOSFET 100 will be broken.

In the MOSFET 100 according to this embodiment, since the electricresistance between the source electrode 12 and the electric fieldreducing region 34 is low, the accumulated holes are likely to move tothe source electrode 12. Therefore, a reduction in the potential of theelectric field reducing region 34 is prevented and the secondarybreakdown is less likely to occur. As a result, secondary breakdownresistance is improved.

It is preferable that the p-type impurity concentration of the electricfield reducing region 34 be higher than the p-type impurityconcentration of the first p well region 26 a and the second p wellregion 26 b. When the p-type impurity concentration of the electricfield reducing region 34 is high, an increase in the width of adepletion layer which extends to the electric field reducing region 34when a reverse bias is applied is prevented and the depletion layer isprevented from reaching the contact region 32. The contact region 32with a high impurity concentration has a high density of crystaldefects. When the depletion layer reaches the contact region 32, thereis a concern that the amount of leakage current flowing between thesource electrode 12 and the drain electrode 14 will increase.

In addition, when the p-type impurity concentration of the electricfield reducing region 34 is high, the width of the depletion layer inthe drift region 24 increases during the application of a reverse biasand the effect of reducing the electric field of the gate insulatinglayer increases.

It is preferable that the p-type impurity concentration of the electricfield reducing region 34 be equal to or greater than two times thep-type impurity concentration of the first p well region 26 a and thesecond p well region 26 b, in order to reduce the amount of leakagecurrent when a reverse bias is applied and to reduce the maximumelectric field strength of the gate insulating layer.

It is preferable that the top of the contact region 32 come into contactwith the first source region 28 a and the second source region 28 b, inorder to increase the contact area of the contact region 32 with thesource electrode 12.

In the semiconductor device manufacturing method according to thisembodiment, the electric field reducing region 34 and the contact region32 are not formed by oblique ion implantation, but is formed by verticalion implantation. Therefore, the number of ion implantation processes isreduced and it is easy to manufacture the MOSFET 100. In addition, avariation in the manufacture of the MOSFET 100 is reduced.

FIGS. 9 to 13 are cross-sectional views schematically illustrating thesemiconductor device which is being manufactured in the semiconductordevice manufacturing method according to the comparative example. FIGS.9 to 13 illustrate a method for manufacturing the MOSFET 900 illustratedin FIG. 7. The description of the same content as that in themanufacturing method according to this embodiment will not be repeated.

Steps up to the formation of the mask member 54 are the same as those inthe manufacturing method according to this embodiment.

Then, the contact trench 52 is formed in the first plane, using the maskmember 54 as a mask (FIG. 9). The contact trench 52 is formed betweenthe first gate trench 50 a and the second gate trench 50 b.

The inclination angle of the side surface of the contact trench 52 withrespect to the first plane is 90 degrees. In other words, the sidesurface of the contact trench 52 is perpendicular to the first plane.

The contact trench 52 is formed by, for example, anisotropic dry etching(FIG. 9).

Then, aluminum (Al) ions which are p-type impurity ions are implantedfrom one side surface of the contact trench 52 into the SiC layer 10,using the mask member 54 as a mask (FIG. 10). A portion 34 a of thep-type electric field reducing region (third region) 34 is formed by theion implantation.

The ion implantation is oblique ion implantation in which an ionimplantation direction is inclined with respect to the line normal tothe first plane. The inclination angle of the implantation directionwith respect to the line normal to the first plane is, for example,equal to or greater than 15 degrees and equal to or less than 45degrees.

Then, aluminum (Al) ions which are p-type impurity ions are implantedfrom the other side surface of the contact trench 52 into the SiC layer10, using the mask member 54 as a mask (FIG. 11). A portion 34 b of thep-type electric field reducing region (third region) 34 is formed by theion implantation.

The ion implantation is oblique ion implantation in which an ionimplantation direction is inclined with respect to the line normal tothe first plane. The inclination angle of the implantation directionwith respect to the line normal to the first plane is, for example,equal to or greater than 15 degrees and equal to or less than 45degrees.

Then, aluminum (Al) ions which are p-type impurity ions are implantedfrom the bottom of the contact trench 52 into the SiC layer 10, usingthe mask member 54 as a mask (FIG. 12). A portion 34 c of the p-typeelectric field reducing region (third region) 34 is formed by the ionimplantation. The ion implantation is vertical ion implantation.

Then, aluminum (Al) ions which are p-type impurity ions are implantedfrom the bottom of the contact trench 52 into the SiC layer 10, usingthe mask member 54 as a mask (FIG. 13). The p-type contact region(fourth region) 32 is formed by the ion implantation. The ionimplantation is vertical ion implantation.

The subsequent steps in the manufacturing method are the same as thosein the manufacturing method according to this embodiment.

As described above, in the method for manufacturing the MOSFET 900according to the comparative example, the oblique ion implantation needsto be performed at least two times in order to form the electric fieldreducing region 34. Therefore, the number of ion implantation processesis greater than that in the method for manufacturing the MOSFET 100according to this embodiment. As a result, MOSFET manufacturing costsincrease.

According to the method for manufacturing the MOSFET 100 according tothis embodiment, the number of ion implantation processes is reduced andmanufacturing costs are reduced.

In the method for manufacturing the MOSFET 900, the electric fieldreducing region 34 on the side surface of the contact trench 52 isformed by oblique ion implantation.

Therefore, the width of a portion of the drift region 24 which isinterposed between the first gate insulating layer 16 a and the electricfield reducing region 34 varies with a variation in the range of ionimplantation. Therefore, there is a concern that a variation inon-resistance will increase.

In addition, since the electric field reducing region 34 is formed by aplurality of ion implantation processes, a variation in, for example,the shape and p-type impurity concentration of the electric fieldreducing region 34 increases.

According to the method for manufacturing the MOSFET 100 of thisembodiment, the electric field reducing region 34 is formed by verticalion implantation. Therefore, the width of a portion of the drift region24 which is interposed between the first gate insulating layer 16 a andthe electric field reducing region 34 does not depend on a variation inthe range of ion implantation. As a result, a variation in on-resistanceis reduced.

According to the method for manufacturing the MOSFET 100 of thisembodiment, since the electric field reducing region 34 is formed by oneion implantation process, a variation in, for example, the shape andp-type impurity concentration of the electric field reducing region 34is reduced.

FIGS. 14 and 15 are diagrams illustrating the function and effect ofthis embodiment. FIGS. 14 and 15 illustrate the simulation results ofthe distance between the trench source region 12 a and the electricfield reducing region 34 and on-resistance.

FIG. 14 is a diagram illustrating simulation parameters. A simulation isperformed while the distance (“L” in FIG. 15) between the trench sourceregion 12 a and the electric field reducing region 34 is changed. Inother words, the distance between the trench source region 12 a and theelectric field reducing region 34 is the difference between the distancebetween the trench source region (first region) 12 a and the first gateinsulating layer 16 a and the distance between the electric fieldreducing region (seventh silicon carbide region) and the first gateinsulating layer 16 a. In other words, the distance between the trenchsource region 12 a and the electric field reducing region 34 is thedistance between the end of an opening portion of the contact trench 52in the first plane and a first-gate-trench-side end portion of theelectric field reducing region 34.

As can be seen from FIG. 15, when the distance between the trench sourceregion 12 a and the electric field reducing region 34 is greater than0.2 μm, on-resistance increases rapidly. Therefore, it is preferablethat the distance between the trench source region 12 a and the electricfield reducing region 34 be equal to or less than 0.1 μm.

According to the method for manufacturing the MOSFET 100 of thisembodiment, the electric field reducing region 34 is not formed byoblique ion implantation, but is formed by vertical ion implantation.Therefore, the distance between the trench source region 12 a and theelectric field reducing region 34 can be zero in principle.

It is preferable that the distance between a second-plane-side endportion of the electric field reducing region 34 and the bottom of thecontact trench 52 be equal to or less than 1 μm, in order to form theelectric field reducing region 34, without using high-energy ionimplantation with a high process cost.

It is preferable that the depth of the contact trench 52 be greater thanthe depth of the first gate trench 50 a and the second gate trench 50 b,in order to form the electric field reducing region 34, without usinghigh-energy ion implantation with a high process cost.

FIG. 16 is a cross-sectional view schematically illustrating amodification example of the semiconductor device according to thisembodiment. A MOSFET 200 according to the modification example differsfrom the MOSFET 100 only in that it includes a first high-concentrationdrift region 64 a and a second high-concentration drift region 64 b.

The n-type impurity concentration of the first high-concentration driftregion 64 a and the second high-concentration drift region 64 b ishigher than the n-type impurity concentration of the drift region 24.The n-type impurity concentration of the first high-concentration driftregion 64 a and the second high-concentration drift region 64 b is, forexample, equal to or greater than two times the n-type impurityconcentration of the drift region 24.

According to the MOSFET 200, the electric resistance of a region whichis interposed between the first gate insulating layer 16 a and theelectric field reducing region 34 and a region which is interposedbetween the second gate insulating layer 16 b and the electric fieldreducing region 34 is low. Therefore, on-resistance is lower than thatin the MOSFET 100.

As described above, according to this embodiment, the maximum electricfield strength of the first gate insulating layer 16 a and the secondgate insulating layer 16 b is reduced and the MOSFET 100 that canimprove the breakdown voltage of the gate insulating layer is achieved.In addition, it is possible to improve the breakdown voltage of the gateinsulating layer and to reduce on-resistance. It is possible to achievethe MOSFET 100 with high secondary breakdown resistance.

Furthermore, according to this embodiment, it is possible to achieve theMOSFET 100 with low manufacturing costs. It is possible to achieve theMOSFET 100 in which a variation in characteristics due to amanufacturing variation is small and characteristics are stabilized.

Second Embodiment

A semiconductor device according to this embodiment is the same as thesemiconductor device according to the first embodiment except that itfurther includes an eight silicon carbide layer of a second conductivitytype which is provided between the first gate insulating layer and thefirst silicon carbide region and a ninth silicon carbide layer of thesecond conductivity type which is provided between the second gateinsulating layer and the first silicon carbide region.

Therefore, the description of the same content as that in the firstembodiment will not be repeated.

FIG. 17 is a cross-sectional view schematically illustrating thesemiconductor device according to this embodiment.

A MOSFET 300 according to this embodiment includes a first p-type region66 a and a second p-type region 66 b.

The first p-type region 66 a is provided between the first gateinsulating layer 16 a and the drift region 24. The first p-type region66 a is provided so as to come into contact with the bottom of the firstgate trench 50 a. The first p-type region 66 a is separated from thefirst p well region 26 a.

The second p-type region 66 b is provided between the second gateinsulating layer 16 b and the drift region 24. The second p-type region66 b is provided so as to come into contact with the bottom of thesecond gate trench 50 b. The second p-type region 66 b is separated fromthe second p well region 26 b.

According to this embodiment, the MOSFET 300 having the same effect asthat in the first embodiment is achieved. Since the MOSFET 300 includesthe first p-type region 66 a and the second p-type region 66 b, themaximum electric field strength of the first gate insulating layer 16 aand the second gate insulating layer 16 b is further reduced. Therefore,the MOSFET 300 that can further improve the breakdown voltage of thegate insulating layer as compared to the first embodiment is achieved.

Third Embodiment

A semiconductor device according to this embodiment is the same as thesemiconductor device according to the first embodiment except that thethickness of the first gate insulating layer between the first gateelectrode and the first p well region is less than the thickness of thefirst gate insulating layer between the first gate electrode and thefirst silicon carbide region and the thickness of the second gateinsulating layer between the second gate electrode and the second p wellregion is less than the thickness of the second gate insulating layerbetween the second gate electrode and the first silicon carbide region.Therefore, the description of the same content as that in the firstembodiment will not be repeated.

FIG. 18 is a cross-sectional view schematically illustrating thesemiconductor device according to this embodiment.

In a MOSFET 400 according to this embodiment, the thickness of the firstgate insulating layer 16 a between the first gate electrode 18 a and thefirst p well region 26 a is less than the thickness of the first gateinsulating layer 16 a between the first gate electrode 18 a and thedrift region (first silicon carbide region) 24. In other words, thefirst gate insulating layer 16 a on the bottom of the first gate trench50 a is thicker than the first gate insulating layer 16 a on the sidesurface of the first gate trench 50 a.

In addition, the thickness of the second gate insulating layer 16 bbetween the second gate electrode 18 b and the second p well region 26 bis less than the thickness of the second gate insulating layer 16 bbetween the second gate electrode 18 b and the drift region (firstsilicon carbide region) 24. In other words, the second gate insulatinglayer 16 b on the bottom of the second gate trench 50 b is thicker thanthe second gate insulating layer 16 b on the side surface of the secondgate trench 50 b.

According to this embodiment, the MOSFET 400 having the same effect asthat in the first embodiment is achieved. Since the first gateinsulating layer 16 a and the second gate insulating layer 16 b whichare provided on the bottoms of the first gate trench 50 a and the secondgate trench 50 b, respectively, are thick, the maximum electric fieldstrength of the first gate insulating layer 16 a and the second gateinsulating layer 16 b is further reduced. Therefore, the MOSFET 400 thatcan further improve the breakdown voltage of the gate insulating layeras compared to the first embodiment is achieved.

Fourth Embodiment

An inverter circuit and a driving device according to this embodimentincludes the semiconductor device according to the first embodiment.

FIG. 19 is a diagram schematically illustrating the driving deviceaccording to this embodiment. A driving device 500 includes a motor 140and an inverter circuit 150.

The inverter circuit 150 includes three semiconductor modules 150 a, 150b, and 150 c having the MOSFET 100 according to the first embodiment asa switching element. The three semiconductor modules 150 a, 150 b, and150 c are connected in parallel to each other to form the three-phaseinverter circuit 150 including three AC voltage output terminals U, V,and W. The motor 140 is drive by an AC voltage which is output from theinverter circuit 150.

According to this embodiment, since the inverter circuit includes theMOSFET 100 with improved characteristics, it is possible to improve thecharacteristics of the inverter circuit 150 and the driving device 500.

Fifth Embodiment

A vehicle according to this embodiment includes the semiconductor deviceaccording to the first embodiment.

FIG. 20 is a diagram schematically illustrating the vehicle according tothis embodiment. A vehicle 600 according to this embodiment is a railwayvehicle. The vehicle 600 includes a motor 140 and an inverter circuit150.

The inverter circuit 150 includes three semiconductor modules having theMOSFET 100 according to the first embodiment as a switching element. Thethree semiconductor modules are connected in parallel to each other toform the three-phase inverter circuit 150 having three AC voltage outputterminals U, V, and W. The motor 140 is driven by an AC voltage which isoutput from the inverter circuit 150. Wheels 90 of the vehicle 600 arerotated by the motor 140.

According to this embodiment, since the vehicle 600 includes the MOSFET100 with improved characteristics, the characteristics of the vehicle600 are improved.

Sixth Embodiment

A vehicle according to this embodiment includes the semiconductor deviceaccording to the fifth embodiment.

FIG. 21 is a diagram schematically illustrating the vehicle according tothis embodiment. A vehicle 700 according to this embodiment is a car.The vehicle 700 includes a motor 140 and an inverter circuit 150.

The inverter circuit 150 includes three semiconductor modules having theMOSFET 100 according to the first embodiment as a switching element. Thethree semiconductor modules are connected in parallel to each other toform the three-phase inverter circuit 150 having three AC voltage outputterminals U, V, and W.

The motor 140 is driven by an AC voltage which is output from theinverter circuit 150. Wheels 90 of the vehicle 700 are rotated by themotor 140.

According to this embodiment, since the vehicle 700 includes the MOSFET100 with improved characteristics, the characteristics of the vehicle700 are improved.

Seventh Embodiment

An elevator according to this embodiment includes the semiconductordevice according to the first embodiment.

FIG. 22 is a diagram schematically illustrating the elevator accordingto this embodiment. An elevator 800 according to this embodimentincludes a basket 610, a counter weight 612, a wire rope 614, a hoist616, a motor 140, and an inverter circuit 150.

The inverter circuit 150 includes three semiconductor modules having theMOSFET 100 according to the first embodiment as a switching element. Thethree semiconductor modules are connected in parallel to each other toform the three-phase inverter circuit 150 having three AC voltage outputterminals U, V, and W.

The motor 140 is driven by an AC voltage which is output from theinverter circuit 150. The hoist 616 is rotated by the motor 140 to moveup and down the basket 610.

According to this embodiment, since the elevator 800 includes the MOSFET100 with improved characteristics, the characteristics of the elevator800 are improved.

In the first to third embodiments, the MOSFET is given as an example ofthe semiconductor device. However, the invention may be applied to aninsulated gate bipolar transistor (IGBT).

In the first embodiment, an example in which the first conductivity typeis an n type and the second conductivity type is a p type has beendescribed. However, the first conductivity type may be a p type and thesecond conductivity type may be an n type.

In the first to third embodiments, an example in which the SiC layer ismade of 4H-SiC has been described above. However, the invention can beapplied to other crystal forms such as 3C-SiC and 6H-SiC.

In the first to third embodiments, it is preferable that the n-typeimpurity be, for example, nitrogen (N) or phosphorus (P). However, then-type impurity may be, for example, arsenic (As) or antimony (Sb). Itis preferable that the p-type impurity be, for example, aluminum (Al).However, the p-type impurity may be, for example, boron (B), gallium(Ga), or indium (In).

In the fourth to sixth embodiments, an example in which thesemiconductor device according to the invention is applied to thevehicle or the elevator has been described. However, the semiconductordevice according to the invention may be applied to, for example, apower conditioner of a photovoltaic power generation system.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the semiconductor device, thesemiconductor device manufacturing method, the inverter circuit, thedriving device, the vehicle, and the elevator described herein may beembodied in a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the devices and methodsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A semiconductor device comprising: a siliconcarbide layer having a first plane and a second plane; a first electrodehaving a first region provided in the silicon carbide layer, a width ofa second-plane-side end portion of the first region being less than awidth of a first-plane-side end portion of the first region, a firstinclination angle of a side surface of the first region with respect toa plane parallel to the first plane being equal to or greater than 60degrees and equal to or less than 85 degrees; a second electrode facingthe first electrode, the silicon carbide layer being interposed betweenthe second electrode and the first electrode; a first gate electrode; asecond gate electrode facing the first gate electrode, the first regionbeing interposed between the second gate electrode and the first gateelectrode; a first gate insulating layer provided between the firstregion and the first gate electrode; a second gate insulating layerprovided between the first region and the second gate electrode; a firstsilicon carbide region of a first conductivity type provided in thesilicon carbide layer; a second silicon carbide region of a secondconductivity type provided between the first silicon carbide region andthe first plane and between the first region and the first gateinsulating layer; a third silicon carbide region of the secondconductivity type provided between the first silicon carbide region andthe first plane and between the first region and the second gateinsulating layer; a fourth silicon carbide region of the firstconductivity type provided between the second silicon carbide region andthe first plane; a fifth silicon carbide region of the firstconductivity type provided between the third silicon carbide region andthe first plane and the first region interposed between the fifthsilicon carbide region and the fourth silicon carbide region; a sixthsilicon carbide region of the second conductivity type provided betweenthe second-plane-side end portion of the first region and the firstsilicon carbide region and between the side surface of the first regionand the first silicon carbide region and having a highersecond-conductivity-type impurity concentration than the second siliconcarbide region and the third silicon carbide region; and a seventhsilicon carbide region of the second conductivity type provided betweenthe first silicon carbide region and the sixth silicon carbide regionand having a lower second-conductivity-type impurity concentration thanthe sixth silicon carbide region, a distance between the seventh siliconcarbide region and the second plane being less than a distance betweenthe second plane and the second silicon carbide region and a distancebetween the second plane and the third silicon carbide region.
 2. Thesemiconductor device according to claim 1, wherein asecond-conductivity-type impurity concentration of the seventh siliconcarbide region is higher than a second-conductivity-type impurityconcentration of the second silicon carbide region and the third siliconcarbide region.
 3. The semiconductor device according to claim 2,wherein the second-conductivity-type impurity concentration of theseventh silicon carbide region is equal to or greater than two times thesecond-conductivity-type impurity concentration of the second siliconcarbide region and the third silicon carbide region.
 4. Thesemiconductor device according to claim 1, wherein the distance betweenthe second plane and the seventh silicon carbide region is less than adistance between the second plane and the first gate insulating layerand a distance between the second plane and the second gate insulatinglayer.
 5. The semiconductor device according to claim 1, wherein thefirst inclination angle is equal to or greater than 65 degrees and equalto or less than 80 degrees.
 6. The semiconductor device according toclaim 1, wherein a second inclination angle of a boundary between theseventh silicon carbide region and the first silicon carbide region withrespect to the plane parallel to the first plane is equal to or greaterthan 60 degrees and equal to or less than 85 degrees.
 7. Thesemiconductor device according to claim 6, wherein the secondinclination angle is equal to or greater than 65 degrees and equal to orless than 80 degrees.
 8. The semiconductor device according to claim 1,wherein the sixth silicon carbide region is in contact with the fourthsilicon carbide region and the fifth silicon carbide region.
 9. Thesemiconductor device according to claim 1, wherein a difference betweena distance between the first region and the first gate insulating layerand a distance between the seventh silicon carbide region and the firstgate insulating layer and a difference between a distance between thefirst region and the second gate insulating layer and a distance betweenthe seventh silicon carbide region and the second gate insulating layerare equal to or less than 0.1 μm.
 10. The semiconductor device accordingto claim 1, wherein a distance between a first contact point between thefirst plane and the side surface of the first region close to the firstgate insulating layer and the first gate insulating layer and a distancebetween a second contact point between the first plane and the sidesurface of the first region close to the second gate insulating layerand the second gate insulating layer are equal to or greater than 0.1 μmand equal to or less than 0.8 μm.
 11. The semiconductor device accordingto claim 1, wherein a distance between a first contact point between thefirst plane and the side surface of the first region close to the firstgate insulating layer and the first gate insulating layer and a distancebetween a second contact point between the first plane and the sidesurface of the first region close to the second gate insulating layerand the second gate insulating layer are equal to or greater than 0.3 μmand equal to or less than 0.6 μm.
 12. The semiconductor device accordingto claim 1, wherein the first gate insulating layer and the second gateinsulating layer includes silicon oxide.
 13. An inverter circuitcomprising: the semiconductor device according to claim
 1. 14. A drivingdevice comprising: the semiconductor device according to claim
 1. 15. Avehicle comprising: the semiconductor device according to claim
 1. 16.An elevator comprising: the semiconductor device according to claim 1.17. A method for manufacturing a semiconductor device comprising:forming a second region of a second conductivity type in a siliconcarbide layer including a first region of a first conductivity type andhaving a first plane and a second plane; forming two first trenches inthe first plane of the silicon carbide layer so as to be deeper than thesecond region; forming a second trench in the first plane of the siliconcarbide layer between the two first trenches, using a mask membercovering the two first trenches as a mask, such that the second trenchis deeper than the second region and an inclination angle of a sidesurface of the second trench with respect to the first plane is equal toor greater than 60 degrees and equal to or less than 85 degrees;implanting ions in the silicon carbide layer from the side and bottom ofthe second trench at an angle of 1 degree or less with respect to a linenormal to the first plane to form a third region of the secondconductivity type; implanting ions in the silicon carbide layer from theside and bottom of the second trench at an angle of 1 degree or lesswith respect to the line normal to the first plane to form a fourthregion of the second conductivity type shallower than the third regionand having a higher second-conductivity-type impurity concentration thanthe third region; forming gate insulating layers in the first trenches;forming gate electrodes on the gate insulating layers in the firsttrenches; forming a first electrode filling the second trench; andforming a second electrode on the second plane.
 18. The method accordingto claim 17, wherein the third region has a highersecond-conductivity-type impurity concentration than the second region.19. The method according to claim 17, wherein a depth of the secondtrench is greater than a depth of the first trenches.